Natural gamma-ray exploration for uranium is carried out as a qualitative indicator of uranium mineralization in an earth formation. A high gamma-ray count rate suggests a mineralized region, but cannot always be used as a reliable quantitative measure of uranium ore grade. This is due primarily to conditions of disequilibrium which exist between the parent uranium-238 and the daughter elements, especially bismuth-214, which emit the bulk of gamma-rays contributing to the natural gamma-ray activity. Disequilibrium has been found to exist when the radioactive daughter elements, through various processes such as leaching, become separated from the parent uranium. If sufficient time has not elapsed between the separation of the uranium parent and the radioactive daughter elements, a relatively low gamma-ray activity may be present at the actual ore body. On the other hand, one can find high natural radioactivity coming from the separated daughter with little or no uranium present.
In addition to the above, other elements, particuarly potassium and thorium, emit natural gamma radiation. This radiation, when detected and recorded, reduces the effectiveness of natural gamma radiation activity as a quantitative measure of uranium ore grade.
One current exploration practice is to drill exploration holes on a widely separated pattern and on a closer pattern after a good show of mineralization is found from natural gamma-ray activity. Core samples are taken from the drilled holes and extensively assayed chemically to quantitatively evaluate the ore deposit. This practice, however, is very expensive. For example, the costs of coring a hole and chemically assaying the cores may be as much as seven to ten times the cost of the exploration hole. Moreover, the presently used technology misses many uranium ore bodies because all natural gamma-ray anomalies suggestive of ore grade mineralization cannot be confirmed because of the prohibitive costs of coring and chemical assaying.
Another current exploration practice is to drill an exploration hole after a good show of mineralization is found from measurement of surface natural gamma-ray activity and run a neutron fission log in the drilled hole. Natural earth formations may be characterized with regard to their uranium content on the basis of neutrons resulting from neutron fission of uranium. When a formation containing an uranium ore is irradiated with neutrons, the uranium nuclei react to neutron bombardment by breaking into smaller nuclear fractions which normally are referred to as fission products. The fission or uranium is attended by the emission of prompt neutrons immediately upon occurrence of the fission reaction and also by the emission of delayed neutrons subsequent to the fission process. The delayed neutrons are emitted by the fission products for an appreciable length of time following the fission reaction.
In U.S. Pat. No. 3,686,503 to Givens et al, there is disclosed a borehole logging system for characterizing the uranium content of natural earth formations surrounding a borehole on the basis of measurements of delayed neutrons resulting from neutron fission of uranium. In U.S. Pat. No. 4,180,730 to W. R. Mills, Jr.. there is disclosed a borehole logging system for characterizing such uranium content on the basis of measurements of prompt neutrons resulting from neutron fission of uranium. In both cases the natural earth formations surrounding the borehole are irradiated with repetitive bursts of fast neutrons and, subsequent to each burst and after dissipation of the original source neutrons, delayed and prompt fission neutrons are detected as indications of uranium content of such formations.